Smooth outer coating for combustor components and coating method therefor

ABSTRACT

A coating and method for overcoating a TBC on a component used in a high-temperature environment, such as the combustor section of an industrial gas turbine. The coating defines the outermost surface of the component and is formed of at least two layers having different compositions. An inner layer of the coating contains alumina in a first silica-containing matrix material that is free of zinc titanate. An outer layer of the coating contains alumina, a glass material, and zinc titanate in a second silica-containing matrix material. The outer layer of the coating has a surface roughness of not greater than three micrometers Ra and forms the outermost surface of the component. The coating reduces the component temperature by reducing the convective and radiant heat transfer thereto.

This application is a Division of U.S. patent application Ser. No.10/710,110, filed Jun. 18, 2004.

BACKGROUND OF THE INVENTION

The present invention generally relates to coatings for componentsexposed to high temperatures, such as the hostile thermal environment ofa gas turbine. More particularly, this invention relates to a smoothouter coating for combustor components of a gas turbine component, inwhich the coating reduces the component temperature by reducing theconvective and radiant heat transfer to the component in the combustorsection of the turbine.

Hot section components of aircraft and industrial (power generation) gasturbine engines are often protected by a thermal barrier coating (TBC),which reduces the temperature of the underlying component substrate andthereby prolongs the service life of the component. Ceramic materialsand particularly yttria-stabilized zirconia (YSZ) are widely used as TBCmaterials because of their high temperature capability, low thermalconductivity, and relative ease of deposition by plasma spraying, flamespraying and physical vapor deposition (PVD) techniques. Air plasmaspraying (APS) is often preferred over other deposition processes due torelatively low equipment costs and ease of application and masking.TBC's deposited by APS are characterized by a degree of inhomogeneityand porosity that occurs as a result of the deposition process, in which“splats” of molten material are deposited and subsequently solidify. Theresulting surface of the TBC is relatively rough, with a surfaceroughness of 250 to 350 microinches Ra (about 6 to 9 micrometers Ra)being typical for YSZ deposited by APS (APSTBC). The inhomogeneity andporosity of a plasma-sprayed TBC enhances the thermal insulatingproperty of the TBC, and thus helps to reduce the temperature of thecomponent on which the TBC is deposited. In regard to infrared (IR)transmissivity, analysis has shown that APSTBC is about 20% to 70%transparent to thermal radiation (wavelengths of about 780 nm to about 1mm) when deposited at typically thicknesses of about 250 to 500micrometers. As a result, the thermal protection provided by APSTBC iscompromised in environments that have high thermal radiation loads, suchas within the combustor section of a gas turbine.

To be effective, TBC systems must strongly adhere to the component andremain adherent throughout many heating and cooling cycles. The latterrequirement is particularly demanding due to the different coefficientsof thermal expansion (CTE) between ceramic materials and the substratesthey protect, which are typically superalloys though ceramic matrixcomposite (CMC) materials are also used. To promote adhesion and extendthe service life of a TBC system, an oxidation-resistant bond coat isoften employed. Bond coats are typically in the form of an overlaycoating such as MCrAlX (where M is iron, cobalt and/or nickel, and X isyttrium or another rare earth element), or a diffusion aluminidecoating. During the deposition of the ceramic TBC and subsequentexposures to high temperatures, such as during turbine operation, thesebond coats form a tightly adherent alumina (Al₂O₃) layer or scale thatadheres the TBC to the bond coat.

The service life of a TBC system is typically limited by a spallationevent brought on by thermal fatigue. In addition to the CTE mismatchbetween a ceramic TBC and a metallic substrate, spallation can bepromoted as a result of the TBC being subjected to substances within thehot gas path of a gas turbine. For example, spallation of TBC fromcombustor components such as liners, heatshields and transition piecescan be accelerated in industrial gas turbines that burn liquid fuel orutilize water injection for NOx abatement.

In view of the above, further improvements would be desirable for theability of TBC on combustor components to reject heat and resistspallation.

BRIEF SUMMARY OF THE INVENTION

The present invention generally provides a coating and method forovercoating a TBC on a component used in a high-temperature environment,such as the combustor section of a gas turbine. The invention isparticularly directed to a coating that reduces the componenttemperature by reducing the convective and radiant heat transfer to thecomponent in the combustor section of an industrial gas turbine.

The coating of this invention defines the outermost surface of thecomponent it protects, and is formed of at least two layers havingdifferent compositions. An inner layer of the coating contains first andsecond alumina particles in a first silica-containing matrix materialthat is free of zinc titanate and consists essentially of silica,silicate and/or mullite. The first alumina particles have a particlesize that is coarser than the second alumina particles. An outer layerof the coating contains third alumina particles having a particle sizedistribution finer than the first and second alumina particles, a glassmaterial, and zinc titanate in a second silica-containing matrixmaterial consisting essentially of silica, silicate and/or mullite. Theouter layer of the coating has a surface roughness of not greater than120 microinches Ra (about 3 micrometers Ra) and, as the outermostsurface of the component, is subjected to the hot combustion gaseswithin the combustor section.

The method of this invention involves preparing first and secondslurries from which the inner and outer layers of the coating areformed. As such, the first slurry is free of zinc titanate and containsthe first and second alumina particles in a first silica-forming bindermaterial, while the second slurry contains the third alumina particles,glass material, and zinc titanate in a second silica-forming bindermaterial. Following deposition of the thermal barrier coating on thecomponent, the first slurry is deposited on the thermal barrier coatingafter which the second slurry is deposited on the inner layer. Theslurry layers formed by the first and second slurries are fired to formthe inner and outer layers, respectively, of the coating, with the outerlayer defining the outermost surface of the component.

As noted above, the coating of this invention reduces the componenttemperature by reducing the convective and radiant heat transfer to thecomponent. In particular, the fine particle size distribution of theouter layer enables the outermost surface defined by the outer layer tobe sufficiently smooth to significantly reduce convective heat transferto the component, and the zinc titanate contained in the outer layerserves to reduce the IR transmissivity of the coating. The bimodalparticle size distribution of the inner layer promotes the chemicalinertness and stability of the inner layer. Furthermore, the inner layeris chemically compatible with the outer layer and the absence of zinctitanate in the inner layer promotes the adhesion of the outer layer tothe component. In addition to the above benefits, the coating of thisinvention improves the spallation and erosion resistance of the TBC, andis therefore capable of significantly extending the life of the gasturbine component protected by the thermal barrier coating.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view through a single annularcombustor structure.

FIG. 2 is a cross-sectional view of the combustor structure of FIG. 1,and shows a multilayer outer coating overlaying a thermal barriercoating in accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in reference to a combustor 12of an industrial gas turbine 10, a portion of which is shown incross-section in FIG. 1. The combustor 12 is one of multiple can-annularcombustors located about the periphery of the turbine 10, and has acan-type liner 14 whose interior defines a combustion chamber of theturbine 10. The liner 14 is inserted into a transition piece 18 withmultiple fuel nozzle assemblies 16 located at the head end of the liner14. Both fuel and water may be injected into the combustion chamberthrough the nozzle assemblies 16, with the injection of water being forthe purpose of reducing combustion temperatures and consequently NOxemissions. The invention is not limited to combustors having theconfiguration shown in FIG. 1, but instead is applicable to othercombustor configurations, such as the well-known annular type.

A thermal barrier coating (TBC) system 20 of a type suitable forthermally insulating the interior surfaces of the liner 14 isrepresented in cross-section in FIG. 2. As shown, the TBC system 20includes a bond coat 24 overlying a substrate 22, which is typically butnot necessarily the base material of the liner 14. Suitable materialsfor the substrate 22 (and therefore the liner 14) include nickel, ironand cobalt-base superalloys, as well as nonmetallic structural materialsincluding ceramic matrix composite (CMC) materials. The TBC system 20further includes a thermal barrier coating, hereinafter TBC 26, thatprovides the thermal protection for the substrate 22. A preferredmaterial for the TBC 26 is an yttria-stabilized zirconia (YSZ), apreferred composition being about 3 to about 8 weight percent yttria,though other ceramic materials could be used, such as alumina,nonstabilized zirconia, or zirconia partially or fully stabilized bymagnesia, ceria, scandia or other oxides. The bond coat 24 may be anoverlay coating such as MCrAlX (where M is iron, cobalt and/or nickel,and X is yttrium or another rare earth element), or a diffusionaluminide coating such as a platinum aluminide.

The TBC 26 is depicted as having been deposited by air plasma spraying(APS), by which “splats” of molten material are deposited on the bondcoat 24. As indicated, the TBC 26 has a degree of inhomogeneity andporosity that typically occurs in coatings produced by plasma spraying.In addition, the surface of the TBC 26 is relatively rough, with asurface roughness of about 200 to 500 microinches Ra (about 5 to 13micrometers Ra) being typical for YSZ deposited by APS (APSTBC). Whiledepositing the TBC 26 by APS is of particular interest to thisinvention, other plasma spraying techniques could also be used, such aslow pressure plasma spraying (LPPS; also known as vacuum plasma spraying(VPS)). The TBC 26 is deposited to a thickness that is sufficient toprovide the required thermal protection for the underlying substrate 22and liner 14.

The bond coat 24 is preferably an NiCrAlY overlay coating and the TBC 26is zirconia stabilized by about eight weight percent yttria (8% YSZ).The bond coat 24 is preferably deposited by APS to a thickness of about0.007 to about 0.010 inch (about 175 to about 250 micrometers) and hasan average surface roughness R_(a) of at least about 320 microinches(about 8 microinches) to promote adhesion of the TBC 26. Though notrequired by the invention, the TBC 26 is depicted as having aconstruction disclosed in commonly-assigned U.S. Pat. No. 6,047,539 toFarmer, whereby the TBC 26 has vertical microcracks 36 that extendthrough at least one-half the thickness of the TBC 26, and the densityof the TBC 26 is preferably at least 90% of theoretical, i.e., containsless than 10% porosity by volume and more preferably less than 8%porosity by volume. A suitable thickness for the TBC is about 760 toabout 2500 micrometers.

While many TBC systems use YSZ deposited by APS as the outermost layer,drawbacks include the roughness of the TBC surface, erosion resistance,and transmissivity to infrared (IR) radiation. Within the operatingenvironment of a gas turbine, surface roughness increases turbulent heattransfer from the hot combustion gases to the component and reducesaerodynamic performance. While surface roughness can be reduced bypolishing, such as tumbling or hand polishing, the final surface finishand thickness of the TBC cannot be closely controlled and the additionalprocessing costs are undesirable. Though crystalline YSZ is veryresistant to erosion, the erosion resistance of a YSZ APSTBC issignificantly reduced as a result of its porosity and microcrackstructure, the result of which is that fine particle bombardmentdislodges small pieces of the TBC. In regard to IR transmissivity,analysis has shown that YSZ is about 20% to 70% transparent to thermalradiation (wavelengths of about 780 nm to about 1 mm) when deposited byAPS to thicknesses of about 250 to 500 micrometers. As a result, thethermal protection provided by YSZ APSTBC is compromised in environmentsthat have high thermal radiation loads, such as within the combustor 10of FIG. 1. Finally, another consideration is the susceptibility of YSZTBC's to attack by CMAS, which is a relatively low melting eutectic thatwhen molten is able to infiltrate conventional TBC and promotespallation during thermal cycling.

To address the above concerns, the TBC 26 in FIG. 2 is overcoated by amultilayer outer coating 28. As the outermost coating on the liner 14,the coating 28 defines the outermost surface 34 of the liner 14 andtherefore also determines the surface roughness of the liner 14. Theouter coating 28 of this invention is also tailored to serve as abarrier to thermal radiation, while also having the advantage of beingmore resistant to erosion and CMAS infiltration than the TBC 26. Theouter coating 28 achieves these features of the invention as a result ofits composition and methods of deposition as described below.

The outer coating 28 is generally an alumina-base silica-bound ceramicmaterial. More particularly, the outer coating 28 contains alumina(Al₂O₃) dispersed within a binder matrix material composed of silica(SiO₂), silicates and/or mullite (3Al₂O₃.2SiO₂), the relative amounts ofwhich will vary depending on the firing temperature and subsequentservice temperatures seen by the coating 28, with greater amounts ofmullite forming at higher temperatures. The coating 28 is depicted ascomprising an inner layer 30 contacting the TBC 26 and an outer layer 32defining the outermost surface 34, with the combined thicknesses of thelayers 30 and 32 being less than that of the TBC 26. The compositions ofthe layers 30 and 32 are tailored for their particular function. Theinner layer 30 is preferably limited to containing alumina in a silicamatrix material, while the outer layer 32 includes alumina as well as aglass and zinc titanate (Zn₂TiO₄) in a silica matrix material. Whilealumina is the preferred constituent of the coating 28, up to about 65percent by weight of the alumina could be replaced by other metaloxides, such as zirconia (ZrO₂), magnesia (MgO), titania (TiO₂), ormullite.

A more particular composition for the inner layer 30 contains about 5 toabout 85 weight percent alumina, more preferably 40 to about 60 weightpercent alumina, with the balance being essentially the silica matrixmaterial. The inner layer 30 is deposited on the TBC 26 in the form of aslurry that is subsequently dried and fired. The slurry is preferablyformulated to contain alumina particles in two discrete particle sizeranges. In such a bimodal size distribution, a suitable particle sizerange for the coarser constituent is about 3.0 to about 6.0 micrometersin diameter. A preferred alumina powder for the coarser constituent hasa particle size range of about 3.0 to about 5.5 micrometers in diameter,and is commercially available under the designation A-14 from ALCOA. Asuitable particle size range for the finer alumina particles is about0.05 to about 0.8 micrometers in diameter. A preferred alumina powderfor the finer constituent has a particle size range of about 0.10 toabout 0.6 micrometers in diameter, and is commercially available underthe designation Baikalox SM8 from Baikowski International Corporation.The SM8 material has an agglomerate size distribution (on a cumulativeweight basis) of 65% below 0.3 micrometer, 78% below 0.4 micrometer, 95%below 0.6 micrometer, and 100% below 1.0 micrometer.

In the preferred size ranges, the finer particles are able to fill thespaces between the larger particles at the surface of the inner layer 30to reduce its surface roughness. Another benefit of the bimodal sizedistribution of the alumina particles is that at very high temperatures,silica within the matrix material of the inner layer 30 preferentiallyreacts with the finer alumina particles to form a mullite phase.

The slurry is prepared by combining the alumina powders with a silicaprecursor and a sufficient amount of carrier liquid to enable the slurryto be applied by spraying. A suitable precursor for the slurry is asilicone such as polymethyl siloxane, a particular example of which is aresin manufactured by GE Silicones under the name SR350, and classifiedas a methylsesquisiloxane mixture of the polysiloxane family. A suitablecarrier liquid is an anhydrous alcohol such as methanol or ethanol,though acetone, isopropyl alcohol or trichloroethylene could be used. Asuitable slurry contains about 40 to about 65 weight percent of thealumina powder (preferably having the two particle size ranges discussedabove), about 1 to about 45 weight percent of the silica precursor, andabout 5 to about 90 weight percent of the carrier liquid. The coarserand finer alumina particles preferably constitute, by weight, about 20%to about 55% and about 20% to about 40%, respectively, of the slurry.After being sprayed on the TBC 26 using any suitable sprayer known inthe art, the composition can be dried at room temperature and then firedto burn off the carrier liquid and yield a substantially homogeneousinner layer 30. A suitable thickness for inner layer 30 is in a range ofabout 0.0003 to about 0.007 inch (about 7.5 to about 180 micrometers).

To achieve the desired surface roughness of not more than 120microinches Ra (about 3 micrometers Ra) for the outermost surface 34,the outer layer 32 must have a smoother surface finish than theunderlying TBC 26. As noted above, the outer layer 32 of the coating 28preferably contains, in addition to alumina and silica, a glass materialand zinc titanate, the latter of which promotes the reflectivity of theouter layer 32 by promoting the Mie-like scattering effect of thecoating 28. To achieve this capability, the zinc titanate content isdispersed in the outer layer 32 of the coating 28. A particularcomposition for the outer layer 32 contains, by weight, about 5 to about85% alumina, about 0 to about 35% zinc titanate, about 0 to about 35% ofthe glass material, and the balance the silica-containing matrixmaterial. A more preferred composition for the outer layer 32, byweight, is about 25 to about 65% alumina, about 10 to about 25% zinctitanate, about 10 to about 25% glass material, and the balance thesilica matrix material.

As with the inner layer 30, the outer layer 32 is deposited in the formof a slurry that is subsequently dried and fired. Contrary to the slurryfor the inner layer 30, the slurry for the outer layer 32 preferablycontains alumina particles in a single particle size range and which arefiner than the alumina particles used to form the inner layer 30. Thealumina particles constitute about 5 to about 80 weight percent of theslurry, more preferably about 25 to about 65 weight percent of theslurry for the outer layer 32. A suitable alumina powder for the outerlayer 32 is commercially available under the designation A-16SG fromALCOA, and has an average particle size of about 0.48 micrometers.

A glass frit, zinc titanate, a silica precursor, and a liquid carrierpreferably make up the balance of the slurry. Glass frit particlesconstitute about 0 to about 35 weight percent of the slurry, morepreferably about 10 to about 25 weight percent of the slurry for theouter layer 32. A preferred glass frit material is a proprietarycomposition commercially available from Vitripak, Inc. under the nameV212, with a particle size of −325 mesh (less than 45 micrometers indiameter). While other glass frit materials could foreseeably be used,such as V55B and V213 glass frit available from Vitripak and 7052 glassfrit available from Corning, the V212 material has been shown to besuitable for having a melting temperature and coefficient of thermalexpansion that are compatible with the superalloy substrate 22 and theoperating environment within a gas turbine. Zinc titanate particlesconstitute about 0 to about 35 weight percent of the slurry, morepreferably about 10 to about 25 weight percent of the slurry for theouter layer 32, with a suitable particle size being −325 mesh (less than45 micrometers in diameter).

The above solid components are combined with an appropriate amount ofsilica precursor and a sufficient amount of carrier liquid to yield aslurry. Similar to the inner layer 30, a suitable precursor for thesilica-containing matrix material of the outer layer 32 is a siliconesuch as polymethyl siloxane, a particular example of which is a resinmanufactured by GE Silicones under the name SR355. This silicone is alsoclassified as a methylsesquisiloxane mixture of the polysiloxane family,but yields less silica when fired than the SR350 silicone used to formthe inner layer 30. A higher silica content is preferred for the innerlayer 30 to promote the yield strength of the inner layer 30, therebyincreasing the compliance of the inner layer 30 to promote strainisolation resulting from CTE mismatch between the TBC 26 and the outerlayer 32. Finally, the same liquid carrier used to form the slurry forthe inner layer 30 can be used to form the slurry for the outer layer32. A suitable slurry contains about 1 to about 45 weight percent of thesilica precursor, and about 5 to about 95 weight percent of the carrierliquid.

After being sprayed on the inner layer 30, the slurry can be dried atroom temperature and then fired to burn off the carrier liquid and yielda substantially homogeneous outer layer 32. The surface roughness of theouter layer 32 is in the range of about 20 to about 120 microinches Ra(about 0.5 to 3 micrometers Ra), preferably not more than 40 microinchesRa (about 1 micrometer Ra), which is significantly smoother than thatpossible for the TBC 26 when deposited by APS. A suitable thickness forouter layer 32 is about 0.0005 to about 0.005 inch (about 10 to about130 micrometers).

As a result of their different compositions, the inner and outer layers30 and 32 define distinct inner and outer zones of the coating 28,respectively, as represented in FIG. 2. The thickness, structure andproperties of the outer coating 28 can be tailored by the firingtemperatures and durations used for each layer 30 and 32. A suitablefiring technique is to heat the sprayed composition at a rate of about10° F. per minute (about 5.5° C./minute) to a maximum hold temperatureof about 800° F. to about 2500° F. (about 425° C. to about 1370° C.).The hold temperature is held for a duration of at least one hour toconvert the precursor to the desired silica-containing matrix materialand at least partially sinter the resulting ceramic constituents of thelayers 30 and 32. The degree to which the layers 30 and 32 are sinteredcan be tailored for the service temperature of the component. In apreferred embodiment, the layers 30 and 32 are not sintered to fulldensity, so that voids (not shown) are present in the coating 28. Thevoids serve to reduce the thermal conductivity of the coating 28 as wellas provide stress relief.

An important feature of the outer coating 28 of this invention is thatit reduces the temperature of the component it protects by reducing theconvective and radiant heat transfer to the component. In particular,the outermost surface 34 defined by the outer layer 32 of the coating 28is sufficiently smooth to significantly reduce convective heat transferto the component, and the zinc titanate contained in the outer layer 32serves to reduce the IR transmissivity of the coating 28. The innerlayer 30 is formulated to be compliant (for strain isolation) andchemically compatible with the outer layer 32, and the absence of zinctitanate in the inner layer 30 has been shown to promote the adhesion ofthe outer layer 32 to the TBC 26. Voids within the at least the outerlayer 32 also potentially serve as radiation-scattering centers tosignificantly reduce heating of the liner 14 by thermal radiation. Thevoids are capable of providing this advantage by having an index ofrefraction different from that of the alumina particles, glass frit,zinc titanate, and silica-containing matrix material. Portions of theradiation propagated through the coating 28 are forward-scattered andback-scattered by the voids, similar to Mie-scattering that occurs whensolar radiation is scattered in all directions by water droplets in theatmosphere. A suitable level of porosity for the outer coating 28appears to be on the order of about 10% porosity, though lesser andgreater levels of porosity are foreseeable. The voids form as a resultof spaces between the alumina particles as well as from thedecomposition of the organic portions of the matrix material precursorand the carrier of the as-deposited slurry coating.

In an investigation leading to the invention, testing was performed withone-inch diameter (about 25 mm) superalloy buttons on which a bond coatof NiCrAlY was deposited by APS to a thickness of about 0.006 inch(about 150 micrometers), over which a TBC of YSZ was deposited by APS tohave a thickness of about 0.005 to about 0.008 inch (about 125 to about200 micrometers. Some of the buttons were additionally coated with atwo-layer outer coating in accordance with the invention. The outercoatings were formed by preparing separate slurries for the inner andouter layers of the outer coatings, as discussed above. The slurrycomposition for the inner layers contained about 316 grams SR350silicone, about 376 gams of the fine SM8 alumina powder, about 516 gamsof the coarser A-14 alumina powder, and about 500 gams of reagentalcohol as the liquid carrier. The slurry composition used to form theouter layers of the coatings contained about 150 grams SR355 silicone,about 500 gams of the A-16SG alumina powder, about 250 gams of zinctitanate, about 250 grams of the V212 glass, and about 500 gams ofreagent alcohol as the liquid carrier. When preparing the slurries,their respective silicone constituents were first dissolved in theliquid carrier, after which the powder materials were added and then themixtures ball milled for about twelve hours.

The slurry compositions were individually applied to the buttons andthen sintered at a temperature of about 1650° F. (about 900° C.) for aduration of about one hour to convert the SR350 and SR355 precursors tothe desired silica-containing matrix materials. The inner layer of theouter coating on each button had a final thickness of about 0.003 toabout 0.008 mils (about 75 to about 200 micrometers) and a finalcomposition of about 79 weight percent alumina with the balanceessentially the silica matrix material. The outer layers wereapproximately about 0.0005 to about 0.005 mils (about 10 to about 125micrometers) thick and contained, in weight percent, about 48% alumina,about 24% glass, about 24% zinc titanate, with the balance essentiallythe silica matrix material.

The buttons were then subjected to a test in which a flame was directedat their coated surfaces, followed by cooling air. The backsides of thebuttons were continuously subjected to cooling air. During the test, inwhich a temperature of about 2550° F. (about 1400° C.) was attained atthe coating surfaces, the buttons protected with the outer coating ofthis invention exhibited backside temperatures of about 1730° F. (about943° C.) compared to about 1830° F. (about 998° C.) fo the buttonscoated only with TBC, for a difference of about 100° F. (about 55° C.).

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art, such as by substituting other TBC, bond coat andsubstrate materials. Accordingly, the scope of the invention is to belimited only by the following claims.

1. A method of reducing convective and radiant heat transfer to acombustor component of a gas turbine, the method comprising the stepsof: depositing a thermal barrier coating on the component; preparingfirst and second slurries, the first slurry being free of zinc titanateand containing first and second alumina particles in a firstsilica-forming binder material, the first alumina particles having aparticle size range of about 3 to about 6 micrometers, the secondalumina particles having a particle size range of about 0.05 to about0.8 micrometers, the second slurry containing third alumina particles, aglass material, and zinc titanate in a second silica-forming bindermaterial, the third alumina particles having a particle size range ofless than the first and second alumina particles; depositing and firingthe first and second slurries to form first and second layers of amultilayer outer coating overlying the thermal barrier coating, thefirst layer being free of zinc titanate and comprising alumina in afirst silica-containing matrix material, the second layer comprisingalumina, the glass material, and the zinc titanate in a secondsilica-containing matrix material; wherein the second layer defines anoutermost surface of the component, the outermost surface having asurface roughness of not greater than 3 micrometers Ra.
 2. A methodaccording to claim 1, wherein the first alumina particles constituteabout 20 to about 40 weight percent of the first slurry.
 3. A methodaccording to claim 1, wherein the second alumina particles constituteabout 20 to about 55 weight percent of the first slurry.
 4. A methodaccording to claim 1, wherein the third alumina particles constituteabout 25 to about 65 weight percent of the second slurry.
 5. A methodaccording to claim 1, wherein the first and second slurries aredeposited to yield the outer coating having a thickness of less than thethermal barrier coating and the outermost surface has a surfaceroughness of not greater than 1 micrometer Ra.
 6. A method according toclaim 1, wherein the first slurry consists of the first and secondalumina particles, the first silica-forming binder material, and acarrier liquid that is eliminated during the firing of the first slurry.7. A method according to claim 1, wherein the second slurry consists ofthe third alumina particles, the glass material, the zinc titanate, thesecond silica-forming binder material, and a carrier liquid that iseliminated during the firing of the second slurry.
 8. A method accordingto claim 1, further comprising depositing a bond coat on the componentprior to depositing the thermal barrier coating.
 9. A method accordingto claim 8, wherein the bond coat has a chemical composition consistingessentially of nickel, chromium, aluminum, yttrium and incidentalimpurities, and the bond coat has an average surface roughness Ra of atleast about 8 micrometers.
 10. A method according to claim 1, whereinthe thermal barrier coating is deposited by air plasma spraying.
 11. Amethod according to claim 10, wherein the thermal barrier coating has achemical composition consisting essentially of zirconia, yttria andincidental impurities.